What can O2 tell us about the climate change in the oceans?

Taka Ito

taka.ito@eas.gatech.eduSchool of Earth and Atmospheric Sciences

Georgia Institute of Technology

Global oxygen cycle

How would CO2 respond to these processes?

CO2 and O2 : Yin and Yang

• Between ocean and atmosphere, which one is the dominant reservoir of oxygen?– O2: 0.5% ocean vs 99.5% atmosphere

• How about carbon dioxide?– CO2: 98% ocean vs 2% atmosphere

Why study oxygen cycling?

• O2 controls many chemical reactions in the seawater and seafloor sediment

• O2 is essential for life

• O2 may inform us about changes in ecosystem and ocean circulation

Photosynthesis and respiration

• In a very approximate form,

– Photosynthesis:• Sun-lit surface ocean• Sink for carbon, source for oxygen

– Respiration: • Happening throughout water column• Sink for oxygen, source for carbon

Global ocean circulation

Sarmiento and Gruber (2006)

Inter-basin contrast

• “Young” North Atlantic Deep Water is high in O2

and low in DIC

• “Old” North Pacific Deep Water is low in O2 and high in CO2

• Upwelling regions have particularly low O2 and high CO2

Sarmiento and Gruber (2006)

Tropical Pacific Oxygen Minimum Zone

• Upwelling region- Transport of low O2 water from below- Upwelling provides nutrients for photosynthesis- Biological O2 consumption in the thermocline

Aug 2006

Image from an ROV off the Oregon coastFraction of living organisms

Vacquer-Sunyer and Duarte [2008]

Impacts on coastal ecosystems

Mortality rapidly increases at the hypoxic condition (Hypoxic = 60 mmol/kg)

Hypoxic event in Oregon coast, 2002

Grantham et al (2004)

• What are the possible causes of low oxygen events?

Is the ocean losing oxygen?

• Last 30 years in California coast– Reason for the growinglow O2 region is not fully

Understood

– Expansion of low-O2

habitat a concern

Bograd et al. (2008)

July 2005Tracy Arm (Sitka), AK

An example: Expansion of Dosidicus gigas habitat in 2000s

British ColumbiaSept. 2005

1984

Long Beach, WAOct 2004

2004Outer Coast, BC

La Jolla Cove, CA.July, 2002

2001

2004

Global changes in oceanic O2

Gruber et al. (2007)

NPIW

• A global change: “ocean deoxygenation”

PDO

Once every 2-8 years

Once every 25-40 years

Impact of climate variability

• Tropical Pacific- Enhanced ocean color

(chlorophyll) variability- Size of OMZ is correlated

with PDO (Deutsch et al 2011)

A conceptual model for sub-surface O2

• Air-sea gas exchange (O2,sfc, set to a constant) • Ocean circulation (W)• O2 loss by respiration (R)• Depth of the thermocline (H)

Steady solution• Long-term change can be explained by the steady

state solution– Set RHS = 0 and solve for O2

• Decreasing oxygen can be explained by– Increasing sea surface temperature (smaller O2,sfc)

– Increasing biological O2 consumption (R)

– Decreasing transport supply of O2 (W)

A possible mechanism for PDO-O2 relationship

Deutsch et al (2011)

• +PDO and high O2

• -PDO and low O2

Summary

• Yin & Yang: CO2 and O2 often show opposite tendency– Photosynthesis and respiration– “Aging” of ocean water masses

• What can O2 tell us about climate and oceans?– Multiple controls: T, circulation and biology– Global de-oxygenation?– Modes of climate variability?